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Credit Card Checksum
From David Shaw's <a href=http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/172845>recipe</a>
def cardLuhnChecksumIsValid(card_number):
sum = 0
num_digits = len(card_number)
oddeven = num_digits & 1
for count in range(0, num_digits):
digit = int(card_number[count])
if not (( count & 1 ) ^ oddeven ):
digit = digit * 2
if digit > 9:
digit = digit - 9
sum = sum + digit
return ( (sum % 10) == 0 )
